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Full text of "Communication Throwing Ethernet Frames OCR"

Low-speed modems are easy to design 

with ICs. They can perform all of the necessary 
functions, from tone generation to signal conversion. 



Fur some engineers, the design of a data modem 
(modulator demodulator) is still a major project. 
But in low-speed applications, in which simple fre- 
quency -shift keying (FSK) suffices, all of the 
olucks m the modem biock diagram can be realized 
by using one or more ICs (Fig. 1). 

Line drivers and receivers that conform to FIA 
specification KS-232 (see box) are available in 
quad Its (foui drivers or receivers per package). 
1 he control logic and signal-converter circuitry 
usually consist of an assortment of standard digi- 
tal It h. With perhaps an up amp included in the 
signal converter to provide it with a frequency- 
discrimination capability. And the main compo- 
nents in the tone generator and the active filters 
arc usually a bunch ot IC op amps. 

Hysteresis needed in line receiver 

Since the line receiver will usually be pm chased 
as a single component, it is more important to 
Know how to buy one than how to build one. 

One important tact to boar in mind is that most 
modems have eight to 16 lines connecting them 
with the computer terminal. To save space, there- 
lore, it ts desirable to use lme-recei ver chips that 
contain several complete receivers per package. 

A second important characteristic to look for is 
high input hysteresis. Without it the line re- 
ceiver will be too susceptible to input noise. This 
point is so important that the Motorola MCT48i)A 
line driver (Fig, 2) has been designed to have 
more than four times as much hystersis as its 
predecessor, the MCT489. The later model has a 
typical turn-on threshold of 2 V, but it doesn't 
tum-off until the input voltage falls below 0.8 V. 
The earlier units had only 250 mV of hysteresis. 

The entii e hysteresis loop is above ground, so 
that the receiver meets the fail-safe requirements 
of RS3-232-C This means the receiver output al- 
ways goes to a mark condition when the input 
sees an open circuit. 

A third desirable characteristic is a provision 



Clay Tatom, Manager, Communications Marketing, Motor- 
ola Semiconductor Products, Inc.. P.O. Box 20906. 
Phoenix. Ariz 85036 



What is EIA spec RS 232? 

The Electronic Industries Association (EIAT 
has standardised a specification to which the 
voltage and the impedance levels of the ONEs 
and ZEROs on the computer side of the modem 
must conform so that any modem can talk to 
any computer with which it might be mated. 
So, the RS-232 specification standardizes the 
voltage and impedance levels at the computer- 
modem interface (not at the modem-telephone 
interface). Two different versions of this speci- 
fication exist: the older RS-2-52 B spec and the 
newer KS-2:'>2-C. Most modems being designed 
today for industrial use conform to the RS-232-C 
spec. The mu.~t important features of this speci 
fication arc given below. 

Electrical Specifications 

Driver output logic levels 

with 3k to 7k load 
Driver output voltage with 

open circuit 
Driver output impedance 

with power off 
Output short circuit 

current 
Driver slew rate 
Receiver input impedance 



Receiver input voltage 

Receiver output with open 

circuit input 
Receiver output with 300 

ohms to ground on 

input 

Receiver output with 

+ 3 V input 
Receiver output with 

-3 V input 



15 V > V„„ > 5 V 
-5V> V UL > -15 V 
j V., ■ < 25 V 

Z > 300 ohms 

IU < -5 A 

dv/dt < 30 V/ M s 
7k ohms > R,„ > 3k 

ohms 
^25 V compatible 

with driver 
mark 

mark 



space 
mark 



RS-232-B +25 
RS-232-C +15 



RS-232-C -15 * 
RS-232-B -25 




50 



Ellciromc Disigis 18. September 2. 1971 



COMPUTER 
TERMINAL 



LINE 
DRIVER 



LINE 




CONTROL 
LOGIC 




FSK 
TONE 
GENERATOR 


RECEIVER 







SIGNAL 
CONVERTER 



ACTIVE 
FILTERS 




1. Each of the blocks in this modem can be realized 
with one or more ICs. Only the tone generator and filters 
will need some passive components. When working with 
the switched telephone network, a Data Access Ar- 
rangement must connect the modem with the network. 



RESPONSE -CONTROL 
INPUT O 




2. This line receiver has over a volt of hysteresis. The 

response-control input can be used to shift the input 
threshold by returning it to an auxilliary supply voltage 
through an external resistor; it can be used to provide 
extra noise filtering by returning it to ground through 
an external capacitor; or it can be used to connect two 
receivers in parallel to the same input line by tying the 
line to them through 8-kn resistors. 



for threshold shifting. This enables the unit to be 
used in the control-logic circuitry as a level 
translator between MOS devices and TTL or DTL 
circuits. 

The control logic itself is usually a section of 
TTL, DTL or MOS logic circuits that control the 
tone generator, FSK, and or signal-conversion 
stages of the modem. The actual logic varies with 
the modem manufacturer, but most use this con- 
trol section to gate the FSK switches on or off 
and to drive the inputs of the line drivers. 

This FSK unit uses four op amps 

Although highly sophisticated modulation tech- 
niques are needed for high-speed data transmis- 
sion, most low-speed systems being built today 
use simple FSK tone generators, such as the one 
shown in Fig. 3. The circuit features an integra- 
tor Schmitt-trigger loop with fourth-order filter- 



ing for the output signal. A pair of dual op amps 
is used to minimize the parts count. Good stability 
is ensured by the zener clipping at the trigger 
input. 

Variable frequencies are generated by changing 
the input transconductance to the integrator 
through a Tee network. Shunt switches may then 
be used to frequency shift from 1070 Hz to 1270 
Hz, for the example shown. The integrator ampli- 
tude is constant with this approach, and its har- 
monics are easily filtered. 

The filter employed is a modified double-sec- 
tion, second-order Butterworth type, with slight- 
ly more peaking than normal to equalize the sig- 
nal amplitudes and still retain maximum 
harmonic suppression. Ordinary Sallen-Key type 
low-pass niters are used since the corner frequen- 
cies are not critical and the relative Q is 1. 

The active-filter section of the modem is 
probably the most controversial of all the blocks. 



Electronic Design 18. September 2. 1971 



51 




0.01 /iF 

He— 





1 kS 


| 10k 




r-*"> l0k 






ii 



1 



5'. 



0.01 /IF 



10k 



6-I5V 



MIOk 



100k 3 



001 /IF ; 




0.01 M_F 



12 k 




H 



^— | O.OOI/iF^ — IFl 
i 1 i* V 1 



1/iF 



~ mmmnmmm&m 



FREQUENCY 
SHIFT 
OUTPUT 
O 



\ 



3. This FSK tone generator has good stability because 
of the zener-diode clipping at its input. The output fre- 
quency shifts from 1070 to 1270 Hz when the input 



BANDPASS 
FILTER 




BANDPASS 
FILTER 



LOW -PASS 




FILTER 









_ru~L 



v7 




COMPARATOR 



6 BIAS 




4. Here's one way to build a low-speed signal converter. 

This circuit is essentially the opposite of a tone gen- 
erator — its output changes logic levels when its input 
frequency changes. 



changes from a mark to a space. Each of the op amps 
is half of an MC1458G. The waveform shows phase con- 
tinuity during tone switching. 

longer has to select resistors, capacitors, and or 
inductors to 1% or 0.1% to achieve his filter 
characteristics. No longer does he have to worry 
so much about component drift with temperature 
either. This is the approach Motorola engineers 
recommend for most active filters in a modem.' 

The signal-conversion block is almost as con- 
troversial as the active-filter block. Most modems 
are built with TTL or DTL logic circuits perhaps 
combined with an operational amplifier which 
provides frequency-discrimination characteristics. 
This stage takes the audio tones coming from the 
active filter and demodulates them to give either 
a ONE or a ZERO depending upon which fre- 
quency tone is present. Usually the output is at 
TTL logic levels. Like the active-filter section, 
designs are so varied that no standard circuit ex- 
ists for this block of the modem, although a 
typical circuit might look like that of Fig. 4. 



Designers of active filters usually have their own 
preferred technique. So long as it works for them, 
they are often not inclined to explore the advan- 
tages of other approaches. 

Motorola has worked with several techniques. 
One is the Sallen-Key method shown in the tone 
generator. Another is the "bi-quadratic" tech- 
nique, which has been around Bell Telephone Lab- 
oratories a long time but has become popular only 
recently, because the low-cost IC op amp made it 
economically feasible. This technique requires 
several active elements instead of only one, but 
this trade-off today usually results in production- 
cost savings as well as other advantages. 

The big advantage of the "bi-quadratic" filter 
is that when the designer uses the several active 
op-amp elements that the design requires, he no 



Don't let the driver slew too fast 

The last block of Fig. 1 that remains to be dis- 
cussed is the RS-232 line driver. As with the line 
receiver, quad packaging is desirable since eight 
to 16 lines usually have to meet the RS-232-C 
interface specification. 

A point worth noting is that RS-232-C specifies 
a maximum slew rate of 30 V. /xs. Some drivers, 
such as the Motorola MC1488, will exceed this 
rate if they are made to work into less than 330 
pF of capacitance. For very short lines, therefore, 
an external capacitor may be needed to roll the 
slew rate down to the 30 V/jus maximum. ■■ 

Reference: 

1. Motorola Integrated Circuits for Modem and Ter- 
minal Systems, 1970. 



52 



ELECTRONIC Dlsign 18. September 2, 1971